The present invention relates to a semiconductor optical amplifier module and an optical communication system, and more particularly to a technology which is effective when applied to semiconductor optical amplifiers for use in long-distance or short-distance, large-capacity optical communication systems such as wavelength-division-multiplexing communication systems.
Wavelength-division-multiplexing communication systems will become the mainstay as a next-generation optical communication system for supporting the future information infrastructure, and their trunk line systems are said to require a transmission rate of 1-10 Tbit/s (see xe2x80x9cResearch and Development Report of Femtosecond Technology (1997)xe2x80x9d, published in March 1997, pp. 431-441, Femtosecond Technology Research Association). The wavelength-division-multiplexing communication systems are described on pp. 101-106 of xe2x80x9cNikkei Electronicsxe2x80x9d(No. 719) published by Nikkei Business Publications, Inc. on Jun. 29, 1998.
The term xe2x80x9cwavelength-division-multiplexing (WDM) communication systemxe2x80x9d means an optical communication system which uses a plurality of optical signals each having a different wavelength and collectively transmits such optical signals via an optical fiber to implement transmission of as many signals as possible per unit time (frequency-division multiplexing of signals). Currently available WDM optical communication systems can transmit on a single optical fiber a multiplexed optical signal having a plurality of 1.55 xcexcm-band different wavelengths at an interval of 0.8 nm (100 GHz). Each wavelength is transmitted at a rate of 10 Gbit/s.
On the other hand, optical amplifiers which perform optical amplification at repeating points in an optical communication system are divided roughly into two types: an optical fiber amplifier and a semiconductor laser amplifier. The optical fiber amplifier uses an optical fiber as an amplifying medium, which is prepared by adding a rare earth element to a part of or to the whole of its core, and by optically exciting the core with a high-output semiconductor laser. The gain wavelength band of the conventional optical fibers ranges from 1.53 xcexcm to 1.56 xcexcm (gain wavelength bandwidth: 30 nm) and from 1.565 xcexcm to 1.6 xcexcm (gain wavelength bandwidth: 35 nm) in the vicinity of the 1.55 xcexcm band, as disclosed in JP-A-10-229238 (laid open on Aug. 25, 1998).
The semiconductor laser amplifier has basically the same structure as a semiconductor laser, and produces an optical gain by injecting a current into an active layer which serves as an optical waveguide (see pp. 618-623 of xe2x80x9cOpticsxe2x80x9d(Vol. 25) published by the Japan Society of Applied Physics in 1996). The active layer has a double heterostructure or a quantum well structure. By changing the material or composition of the active layer, one can fabricate a plurality of amplifiers each dedicated to a wavelength to be amplified. Unlike a semiconductor laser, the optical input/output facets of the semiconductor laser amplifier must have very low reflectivity in order to suppress laser oscillation. In this case, the gain bandwidth is said to be in the order of several THz (about tens of nanometers) (see p. 1227 of xe2x80x9cApplied Physicsxe2x80x9d (Vol. 59, No. 9) published by the Japan Society of Applied Physics in 1990).
The conventional optical amplifiers have limited gain wavelength bandwidths ranging from about 50 nm to 60 nm, which are not usable as optical amplifiers for WDM optical communication systems having a transmission rate of 1-10 Tbit/s. Proposed as a solution to this problem of limited gain bandwidths is a method of using a demultiplexer/multiplexer comprising directional couplers and Y-branch waveguides to amplify input signals each having a different wavelength at a plurality of optical amplifying regions each having a different gain wavelength bandwidth, respectively (JP-A-7-176824 laid open on Jul. 14, 1995). However, this publication discloses only the method of amplifying two demultiplexed signals respectively having wavelengths of 1.30 xcexcm and 1.55 xcexcm, but proposes no technology for amplifying a wavelength-multiplexed signal containing 100 or more demultiplexed signal components.
To implement a transmission rate of 1-10 Tbit/s, the conventional WDM optical communication systems is required to combine 100 to 1000 wavelengths into a wavelength-multiplexed signal, and their gain wavelength band is in the range of 80-800 nm. Therefore, optical amplifiers to be employed in WDM optical communication systems must have a band broad enough to satisfy this gain wavelength band (80-800 nm).
Further, the WDM optical communication systems use wavelengths arranged at narrow intervals, e.g., at about 1 nm. For the demultiplexing/multiplexing of an optical signal containing a large number of wavelength components arranged at narrow intervals, the systems employ many directional couplers and Y-branch waveguides as mentioned before, and this has imposed a problem of increased element size.
On the other hand, each semiconductor optical amplifier addresses (1) a problem of reduced gain (gain saturation) due to the intensity of the amplified light increasing and the carrier density within the active layer of the amplifier thereby reducing when the optical intensity of an incident signal is increased, and (2) a problem of phenomenon (four wave mixing) which produces a new optical signal due to wavelength conversion caused by nonlinear optical effects during transmission through a semiconductor of a plurality of signals each having a different wavelength.
One object of the present invention is to provide a broad band semiconductor optical amplifier module and an optical communication system capable of dealing with Tbit/s-order high-speed transmission which is increasingly required for transmitting video information and moving picture information used in the rapidly proliferating Internet.
Another object of the present invention is to provide a semiconductor optical amplifier module and an optical communication system exhibiting a lesser degree of gain reduction caused by gain saturation.
Still another object of the present invention is to provide a semiconductor optical amplifier module in which four wave mixing is hard to occur.
Even another object of the present invention is to provide a broad band and downsized semiconductor optical amplifier module exhibiting a lesser degree of gain reduction, which is capable of simultaneously amplifying as many as 100 to 1000 wavelength components or more arranged at a narrow interval of about 1 nm or less.
According to one aspect of the present invention, an optical input signal contains, e.g., as many as 100 to 1000 wavelength components or more, and such an input signal is separated by a demultiplexer into a plurality of demultiplexed signals each having a different one of the wavelength components. The plurality of demultiplexed signals are amplified by a plurality of spatially separated, independent amplifiers, respectively, and the amplified demultiplexed signals are combined by a multiplexer to produce a single amplified optical output signal having the plurality of wavelength components. The demultiplexer, the amplifiers, and the multiplexer are formed on a single semiconductor substrate, and the optical amplifiers are optically coupled to the demultiplexer and the multiplexer. The plurality of amplifiers have a multiple-quantum-well structure, at least one being provided on the semiconductor substrate for each of the wavelength components.
The above and other novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.